1. Field of the Invention
The present invention relates to a variable valve timing control device for an internal combustion engine which controls the valve timing of the engine in accordance with the operating condition thereof.
2. Description of the Related Art
A variable valve timing control device for controlling the valve timing, i.e., the timing of inlet valves and/or exhaust valves of an engine is commonly known. The variable valve timing control device is used for setting the valve timing in accordance with the operating conditions of the engine such as the engine speed and the engine load in order to improve the fuel consumption, the engine output and the exhaust emission of the engine.
In the variable valve timing control device, the maximum operating speed of the device for changing the valve timing sometimes limits the response of the engine during a transient between the operating conditions. For example, when the difference between the valve timing settings for a high load operation and for a low or medium load operation of the engine is large, the variable valve timing control device must change the valve timing by a large amount when the engine load is increased from a low or medium load to a high load rapidly (such as, when the vehicle is accelerated). In this case, if the maximum operating speed of the variable valve timing control device is not sufficiently large, a relatively long time is required for the device to change the valve timing to new setting, i.e., the device cannot follow the speed of the change in the engine load. This causes a reduction in the output of the engine during the acceleration and thus a deterioration in acceleration of the vehicle.
Japanese Unexamined Patent Publication (Kokai) No. 60-190610 discloses one method for solving the problem caused by the delay in the response of the variable valve timing control device. The variable valve timing control device disclosed in JPP '610 sets a target value of the valve timing of the engine in accordance with the engine speed and the engine load, and generates a drive signal which has an intensity corresponding to the amount of the deviation of the actual valve timing from the target value of the valve timing. An actuator of the valve timing adjusting mechanism of the device in JPP '610 is driven by the drive signal generated by the variable valve timing control device. Therefore, the operating speed of the valve timing adjusting mechanism becomes higher as the intensity of the drive signal increases, i.e., as the deviation of the actual valve timing from the target value increases. Further, in JPP '610, the target value of the valve timing itself is increased by a predetermined amount during the acceleration of the vehicle. This causes the amount of the deviation of the actual valve timing from the target value to become larger during the acceleration of the engine than during the steady operation of the engine. Therefore, the operating speed of the variable valve timing control device of JPP '610 increases during the acceleration, and the response of the device in changing the valve timing during acceleration is improved.
However, the maximum operating speed of the valve timing adjusting mechanism is limited by the mechanical construction of the mechanism. Therefore, in the variable valve timing control device in JPP '610, it is not possible to obtain the operating speed of the valve timing adjusting mechanism larger than that limited by the mechanical construction of the mechanism even though the intensity of the drive signal is increased. Thus in the variable valve timing control device in JPP '610, though the intensity of the drive signal increases during the acceleration of the engine, the speed of change of the valve timing is limited. Therefore, when the amount of the change in the valve timing is large (such as during acceleration from a low or a medium load operation of the engine), a delay in changing the valve timing occurs during the acceleration of the engine.
In the device in JPP '610, it is possible to improve the response of the device by increasing the maximum operating speed of the valve timing adjusting mechanism. However, it is sometimes difficult to increase the operating speed of the valve timing adjusting mechanism from a practical point of view. For example, when a hydraulic actuator is used for driving the valve timing adjusting mechanism, the maximum operating speed of the mechanism is determined by the capacity of the hydraulic cylinder of the actuator and the maximum flow rate of the working fluid fed to the hydraulic cylinder. Therefore, in order to increase the maximum operating speed of the valve timing adjusting mechanism, the capacities of the hydraulic cylinder and the hydraulic supply system (such as a hydraulic pump, connecting piping and control valves) must be increased. Since this causes an increase in the size and the manufacturing cost of the whole system, it is difficult to increase the maximum operating speed of the valve timing adjusting mechanism. When a stepper motor is used as the actuator of the valve timing adjusting mechanism, same is true since the sizes of the motor and an electric supply system, such as a battery and an alternator, must be increased.
SUMMARY OF THE INVENTION
In view of the problems set forth above, the object of the present invention is to improve the response of the variable valve timing control device without increasing the size and manufacturing cost of the device.
The above object is achieved by the variable valve timing control device according to the present invention which comprises an engine speed detecting means for detecting the rotating speed of the engine, an engine load detecting means for detecting the load on the engine, an optimum full-load valve-timing calculating means for calculating an optimum valve timing of the engine when the engine is operated at a full load at the engine speed detected by the engine speed detecting means, a valve timing setting means for determining a setting value of the valve timing of the engine based on the engine speed detected by the engine speed detecting means and the engine load detected by the engine load detecting means in such a manner that the amount of deviation of the setting value of the valve timing from the optimum full load valve timing does not exceed a predetermined limiting value, and a valve timing adjusting means for adjusting the valve timing of the engine to the setting value determined by the valve timing setting means.
Usually, the valve timings of the respective operating condition are decided based on the optimum valve timing during a low speed and low load operation of the engine. Namely, the optimum valve timing during a low speed and low load operation is used as a reference valve timing, and the valve timings of the respective operating conditions are defined by an amount of deviation thereof from the optimum valve timing during a low speed and low load operation of the engine. Therefore, the valve timing during a low load operation and the valve timing during a high load operation sometimes differ by a large amount. Thus, it is required to change the valve timing by a large amount when the engine load changes from a low load to a high load. Since the maximum operating speed of the valve timing adjusting mechanism is limited, this causes a delay in changing the valve timing.
In one aspect of the present invention, the valve timing for the respective operating conditions (i.e., the combinations of the engine speed and the engine load) are determined based on the optimum valve timing for the full load operation at the respective engine speed. Namely, the optimum valve timings for the full load operations at the respective engine speeds are used as reference valve timings of the respective engine speeds. Further, the valve timings of the respective operating conditions are determined in such a manner that the amount of the deviations thereof from the reference valve timing do not exceed a predetermined value. Therefore, when the engine load is increased from a low load to a full load, the amount of the change in the valve timing, hence the amount of the operation of the valve timing adjusting mechanism, is always smaller than the predetermined amount. Since the maximum amount of the change in the valve timing is limited to an appropriate value, the time required for the valve timing adjusting mechanism to change the valve timing becomes shorter, i.e., the delay in the response for changing the valve timing can be minimized without increasing the maximum operating speed of the valve timing adjusting mechanism.
Further, according to another aspect of the invention, there is provided a variable valve timing control device comprising an engine speed detecting means for detecting a rotating speed of the engine, an engine load detecting means for detecting a load of the engine, an optimum full-load valve-timing calculating means for calculating an optimum valve timing of the engine when the engine is operated at a full load at the engine speed detected by the engine speed detecting means, a valve timing setting means for determining a setting value of the valve timing of the engine based on the present engine speed detected by the engine speed detecting means and the present engine load detected by the engine load detecting means in such a manner that the setting value becomes an optimum valve timing decided by the present engine speed and the present engine load, a correcting means for correcting the setting value of the valve timing in such a manner that the amount of deviation of the setting value of the valve timing from the optimum full load valve timing does not exceed a predetermined limiting value when the amount of the deviation becomes larger than the predetermined limiting value, and a valve timing adjusting means for adjusting the valve timing of the engine to the setting value after it is corrected by the correcting means.
In this aspect of the present invention, the amounts of the deviations of the valve timings of the respective operating conditions from the optimum valve timings of the full load operation at the respective engine speeds are always less than or equal to a predetermined value. Therefore, the delay in the response for changing the valve timing can be minimized without increasing the maximum operating speed of the valve timing adjusting mechanism.